Paramyxoviruses are thought to multiply exclusively in the cytoplasm of a host cell. Progeny virus particles are formed by budding from the plasma membrane, a process orchestrated by matrix (M) protein. The M protein probably interacts with the plasma membrane, the cytoplasmic domain of the viral glycoproteins and the viral nucleocapsid. However, the specific interactions and the mechanisms by which these components are gathered together to initiate budding are not understood. In addition, a recent unexpected finding may point to a new role for the M protein in its relationship with its host cell: the M protein is the only viral protein produced by Newcastle disease virus (NDV) which accumulates in the nucleus, specifically the nucleolus, of an infected cell. Interestingly, NDV is also the most efficient paramyxovirus at inhibiting host protein synthesis. In this study, both the assembly and nuclear targeting of the M protein of NDV will be examined. The interaction between the M protein and the viral fusion (F) glycoprotein will be dissected: 1) genetically, by pinpointing the M protein mutation in mutants defective in this interaction; and 2) physically, by defining the portion of the F cytoplasmic domain peptide which interacts with the M protein and vice versa. The nuclear and nucleolar targeting of the M protein will be dissected: 1) genetically, by transient expression of deletion and site-specific mutants of the M protein gene and analysis of the effects on M protein targeting; and 2) functionally, by quantifying virus production and the inhibition of host protein synthesis in enucleated cells and in a stably transfected cell line expressing the M protein. With a better understanding of the virus assembly process and the viral inhibition of host protein synthesis, therapies might to targeted to control paramyxovirus infections. Such therapies are particularly important for diseases such as bronchiolitis or pneumonia in infants, caused by another paramyxovirus, respiratory syncytial virus. No safe, effective vaccine has been developed for this infection, despite intensive efforts. Even if such a vaccine is developed, it may not be efficacious during the period of greatest risk, the first six months of life. A better understanding of paramyxovirus pathogenesis and assembly might provide clues for alternative treatments to control virus infection and dissemination.